Polyester resins derived from polycarboxylic acids and polyhydric alcohols have been widely used as resinous vehicle in coatings, because an excellent film with toughness and other desirable properties can be obtained therefrom. Irrespective of the polymerization degree, the ester bondings contained are resistive against hydrolysis, which in turn, is believed to be one of the reasons and advantages of using this polymer as resinous component.
In coatings, and especially in ship bottom paint, from the standview that if the coat whose surface is full of undulation is gradually hydrolyzed or dissolved in sea water during sailing, thereby resulting a comparatively flat bottom surface, fuel cost would be greatly economized and furthermore, the tendency for the sea livings as acorn shell or the like to stick to the bottom of ship would be, from the nature of things, greatly reduced, hydrolyzable resinous materials have become the object of public attention of late years. However, very unfortunately, attempts at achieving such polyester material have all ended in failure partly due to the conflicting requirements of moderate stability and gradual decomposition of the material in sea water. Polyester is, as is well known, composed of water soluble polyhydric alcohol and polycarboxylic acid which is also relatively soluble in water due to the presence of polar carboxyl groups. Therefore, if some thing is devised to decompose said polymer into lower molecular segments or it constitutional components in an appropriate rate, then the so-called self-polishing would be realized with this material. The inventors, having the above in mind, have made endeavors to get polyester which is relatively stable in water, but is decomposed or hydrolyzed gradually under sailing conditions. Surprisingly, it was found that in the preparation of polyester by the polycondensation of polycarboxylic acid and polyhydric alcohol components, if a polyhydric alcohol having the metal ester bonding of the formula: ##STR1## wherein M stands for metallic element and l is an integer corresponding to the valency of said metal M, is used as at least part of said polyhydric alcohol component, a number of said metal-ester bondings can be introduced into the polyester chain, and thus obtained polyester is very stable against hydrolysis under neutral condition, but is gradually decomposed and hydrolyzed under weak alkaline conditions, at the sites of said metal-ester bondings, thereby liberating metal ions and giving water soluble segments with carboxyl groups. Sea water and sailing will offer optimum hydrolysis conditions to this polymer. On the basis of these findings, the inventors have completed the invention.